In the manufacture of integrated electronic devices such as integrated circuits or liquid crystal displays utilizing either direct or active matrix addressing, photolithographic techniques are generally used. Photolithography permits the formation of device elements or structures such as device semiconductor regions, device electrodes, pixel electrodes, and address lines, for example. Photolithography generally entails exposing a film of light sensitive material, such as a photoresist, to a pattern of radiation, such as ultraviolet light, to render the exposed or unexposed film portions soluble to a developer, depending on whether a positive or negative photoresist is utilized. The soluable portions are then selectively removed, for example by etching, to expose selected portions of an etchable layer therebeneath. The etchable layer is then etched in a suitable wet etchant or by a dry etch process, such as by a reactive ion plasma, to form the desired circuit elements from the etchable layer. Because many different kinds of elements make up a finished circuit or display, the photolithography steps must be repeated for many different layers of etchable material. Also, a patterned mask or reticle is often used to derive the pattern of radiation. Hence, many different masks, each corresponding to the pattern of a respective layer, are generally required.
Because photolithographic feature sizes can be rather small in dimension, precision alignment of the various masks for each layer with the patterns of previous layers is absolutely required. The image of the mask which provides the pattern of radiation on the light sensitive film surface must be optically sharp. In accordance with the prior art, there are three basic techniques to facilitate mask alignment and optically sharp exposure.
One such technique is contact printing. Contact printing involves placing a mask in contact with the radiation sensitive film, such as a photoresist, and applying pressure to remove warp effects between the mask and the photoresist. While this can produce excellent focus, the substrate and the mask can be damaged by the contact therebetween and by motion required for aligning the mask with previously patterned layers. In addition, residual substrate material can be transferred to the mask which can cause subsequent printing errors.
Another technique provided by the prior art is proximity printing. In proximity printing, contact damage to the substrate or mask and residual material transfer from the substrate to the mask can be avoided by maintaining a small gap between the adjacent photoresist and mask surfaces during mask alignment and exposure. Proximity printing does not always provide adequate resolution, and experience has shown that the maximum gap is typically 20 to 50 microns assuming the exposing light source is well collimated and the features being exposed are 5 microns in width or larger. This can be a problem, especially for large area applications, where it is difficult to adequately control the flatness of the substrate and the mask.
A further technique known in the art is projection printing. In projection printing, rather expensive projection systems including high cost lenses or spherical mirrors are utilized to insure optimum focusing of an image of the mask onto the photoresist surface. Most lens systems use demagnification and produce very high resolution images over a maximum diameter of only about 1 inch. As a result, step and repeat exposure is generally required for most exposure processes. Spherical mirrors have been made which can expose areas as large as six inches in diameter, but they involve very complex optical and mechanical systems and still require step and repeat operation for large area applications, such as in making liquid crystal displays.
In addition to the foregoing, in projection systems, mercury arc lamps are generally used to provide the required actinic radiation for exposing the photoresist. Such mercury arc lamps provide light at various frequencies known as spectral lines. In projection systems, it is necessary to filter out all of the mercury arc lamp radiation except for one spectral line in order to achieve maximum resolution. Accordingly, a substantial portion of the mercury arc lamp power which would expose the photoresist is filtered out in these systems and not used. As a result, this lenghtens the total amount of time required to properly expose the photoresist.